Stepper Motors

Do you think there’s any chance we’ll ever see steppers on the list of FRC-legal motors?

Open control loops are a whole lot easier than closed ones, and I’ve always felt steppers are a natural solution for a large portion of the FRC problem-space that is currently occupied by PID or bang-bang control loops.

This has been discussed before.

Steppers are really hard to use effectively, it would just widen the gap between the better engineering teams and the less experienced.

It uses open loop control so if it loses steps they stay lost and can accumulate. In an FRC environment there are just too many opportunities to loose steps and it would likely end up being a nightmare for all but the best teams.

An encoder versaplanetary stage accomplishes the same result in a more robust way without changing the rules. I’ve seen clever ways to attach encoders to versaplanetaries so even this isn’t really necesary. (a half inch hex encoder would be cool though)

Combined with the above concerns (on missed counts), stepper motors are pretty awful in terms of power to weight.

I do not think this is really the case. If you choose the wrong stepper for the job, yes, you are going to have problem with skipped steps and accumulated errors. But it is not that hard to pick a stepper with the appropriate torque for a given mechanism.

For many manipulators which require small, precise movements, I think a stepper is far easier to use than a DC motor in a closed control loop. Less complexity, fewer failure points.

Please, walk through the selection of a stepper motor and brushed motor for an elevator for this years game. You’ll see that you end up with a giant stepper motor (or you’ll leave yourself vulnerable to missing counts due to friction/wind blowing/jammed tote/whatever).

Electrical components become legal when enough of them are donated to FIRST to put an adequate amount of them in each kit of parts. If you want to use an electrical component, find a company willing to donate enough of them.

I don’t think that steppers should be restricted. If someone knowledgeable sees an opportunity to use one effectively than why not let them? It may just be hard to imagine how to use them in FRC simply because we haven’t ever seen it.

Clearly, steppers are not suitable for the entire problem space of FRC motion control. You’re not going to lift a tote with one.

That doesn’t mean that they aren’t very well-suited for certain parts of it. Not all of FRC is lifting totes.

The problem is that teams that can already effectively use them are likely already dominant and this would allow them to be more dominant. Or you get the consensus that it’s not useful and end up with more throttle motors, but with their own entire section in the manual.

It also has the very real potential to be misused by less experienced teams and cause massive problems when it doesn’t work properly.

IMHO it’s a can of worms that does not need to be opened.

If I understand correctly, stepper motors require twice as many control signals as a brushed motor. So to save an encoder feedback you add another control circuit - with uncertainty if the torque isn’t sufficient to make the step. It sounds like a step backwards, or at best sideways.

Not necessarily. Plenty of people use them for other robots and things like 3d printers or CNC routers as well as in industrial apps so there should be a large group to draw knowledge from. Plus I’d bet there’s a good bit of overlap between the above the FIRST community. Thus I would imagine several teams who aren’t “dominant” could acquire sufficient knowledge to use the technology effectively.

That might be a desirable trade off you never know. Or a co-processor could generate the singles from instructions sent over via serial or whatever. Options are options and more is more fun.

Hardware and library support trivializes much of this. You can run a stepper off an arduino pretty easily (online tutorials and libraries abound). If FRC were to offer support for common stepper drivers in their control system library (which would probably be the only way this would happen given FRC rules re: co-processor controlled mechanisms), it’d be even easier.

I’d rather see the specs on allowable servos to be opened up.

Ha, sad, but true, I am sure that is legitimately one of the only deciding factors.

On a related note anyone want a bag of discontinued throttle motors :stuck_out_tongue:

Now brushless motors in general… THAT would be a step forward. But steppers, in specific I don’t think would be of much benefit to anyone.


More problematic than the motors are the controllers. You would need a source(s) stable enough to provide the same controller through one or multiple seasons for all the teams. The controller would have to interface with the roborio in such a way that the enable/disable is robust.

I’ll be honest, I’ve spent some time trying to source a brushless motor for some non FRC projects and it’s mildly daunting to find a motor and a controller. And then interfacing with them is a whole other issue (though I’ve found one that I can press fit WCProducts pinions (for bag motors) onto and do decent reductions)

I don’t know, I’ve never had that problem. ESCs for standard brushless motors seem pretty easy to come by once you know what you’re looking for. I’ll second the gear problem though; you tend to have to use set screws or press fits to keep stuff on the shaft.
SDP-SI sells “fairloc” hubs for their gears, which are like integrated shaft collars that allow you to leave the shaft unmarred. Something like that would be good for interfacing a brushless motor.

Low cost, sensored, robust, controllers that go both directions without having to come to a full stop first?

The sensor part is really critical for brushless motors. Without a sensor, you get much less torque at low speeds, and you can sometimes stop in a place that you can’t start from. I do think that brushless motors will eventually replace the motors we use now in FRC.